-#'
-#--------------------------------------
-# Determine if a number is a valid ISBN number, according to length
-# of 10 digits and valid checksum
-sub checkvalidisbn {
- use strict;
- my ($q)=@_ ; # Input: ISBN number
-
- my $isbngood = 0; # Return: true or false
-
- $q=~s/x$/X/g; # upshift lower case X
- $q=~s/[^X\d]//g;
- $q=~s/X.//g;
-
- #return 0 if $q is not ten digits long
- if (length($q)!=10) {
- return 0;
- }
-
- #If we get to here, length($q) must be 10
- my $checksum=substr($q,9,1);
- my $isbn=substr($q,0,9);
- my $i;
- my $c=0;
- for ($i=0; $i<9; $i++) {
- my $digit=substr($q,$i,1);
- $c+=$digit*(10-$i);
- }
- $c %= 11;
- ($c==10) && ($c='X');
- $isbngood = $c eq $checksum;